The present invention relates to an improvement in power driven rotary tools such as screwdrivers and drills, and more particularly to an improvement in such a rotary tool having a torque adjusting mechanism for adjusting driving torque to be transmitted to a spindle of the tool to a fixed value.
Power tools having a torque adjusting mechanism are disclosed, for example, in Japanese Laid-open Utility Model Publications Nos. 59-143670 and 63-30476.
Both of the prior art power tools include sliding clutch members provided between an output shaft of the motor and a spindle of the power tool, and the pressing force between the sliding clutches is adjusted so as to vary the value of torque which causes the clutch members to slide, that is, maximum torque to be transmitted to the spindle of the power tool.
In the prior art power tools as described above, the pressing force between the sliding clutch members is incrementally or steppingly adjusted, so that the maximum torque to be transmitted to the spindle of the power tool is steppingly varied to a torque of, for example, 20 kg, 30 kg or 40 kg.
Such a stepwise adjusting means of the prior art involves a practical problem. Specifically, a user may require, for example, a torque of 25 kg in one case, and a torque of 38 kg in another case. If the required torque is fortunately one of the steppingly adjustable torques, the prior art is sufficiently effective to meet the required torque. However, if any torque intermediate two adjacent steppingly adjustable torques is required, the prior art cannot provide the required torque.
For this purpose, various kinds of tools, for example, for 25 kg torque and 35 kg torque could be prepared, but this would reduce the advantage of mass production.